1. Field of the Invention
This invention relates to Liquid Crystal Display (LCD) and in particular to LCDs of the transflective type.
2. Description of the Related Art
LCDs can be classified based upon the source of illumination. Reflective type displays are illuminated by ambient light that enters the display from the front and the peripheral side. A reflective surface, such as an aluminum or silver reflector placed in or behind the reflective display, reflects the light to illuminate the reflective display. Although reflective displays meet the need for low power consumption, the displays often appear rather dark and are therefore difficult to read. In addition, the ambient light from the front and the peripheral side is not always enough to illuminate the reflective displays, and, thus, the application of the reflective displays will be limited.
In applications where the intensity of ambient light is insufficient for viewing, supplemental lighting, such as a backlight assembly, is used to illuminate the display. Although supplemental lighting can illuminate a display regardless of ambient lighting conditions, it is an expensive drain on battery life. Thus, the batteries on portable computers, for example, must typically be recharged after 2 to 4 hours of continuous backlight use. In applications where the intensity of ambient light is very strong, e.g., under an outdoor burning sun, the transmissive image illuminated only by the backlight assembly is insufficient for viewing because of poor contrast.
In an attempt to overcome the above described drawbacks of reflective and transmissive displays, some electronic displays have been designed to use ambient light when available and backlighting only when necessary. This dual function of reflection and transmission leads to the designation, xe2x80x9ctransflectivexe2x80x9d. Transflective LCDs are a dual mode display device. These devices operate either with the available ambient light in a reflective mode or with an internal backlight in a transmissive mode.
FIG. 15 illustrates a conventional transflective LCD. In the reflective mode, the ambient light 10 passes through the outside polarizer 20, and then passes into the LCD cell 30. Generally, the LCD cell 30 consists of two opposing glass substrates 32, 34 with a liquid crystal layer 36 sandwiched therebetween. Typically, the substrate 34 is provided with a plurality of pixel regions arranged as a matrix with a switching element such as TFT and a pixel electrode (not shown) formed at every pixel region. The substrate 32 is provided with color filters for displaying colors and a common electrode (not shown). While the liquid crystal layer having positive dielectric anisotropy is possible, for the sake of simplicity, it will be assumed that the liquid crystal layer has the more popular negative dielectric anisotropy. Thus, when the switching element is in the xe2x80x9conxe2x80x9d state, the liquid crystal layer has no effect upon the light passing through it. When the switching element is xe2x80x9coffxe2x80x9d, the light passing though the liquid crystal layer will be altered in some way, depending upon the nature of the light and the type of LCD. The light 10 passing through the LCD cell 30 proceeds through the inside polarizer 40 to the transflective film 50 serving as a reflector of ambient light and a transmitter of light from the backlight. At the transflective film 50, a portion of the light is reflected. The reflected light 10 then returns through the inside polarizer 40, through the LCD cell 30 and finally through the outside polarizer 40 to a viewer.
Since the transflective film 50 is placed outside the LCD cell 30, reflections from the interface between the liquid crystal layer 36 and the glass substrate 34 as well as the transflective film 50 will form a multiple image, which can cause serious problem depending on the thickness of the glass and adversely affect the display resolution. Furthermore, in the reflective mode, the light passes through the polarizers 20 and 40 twice; however, in the transmission mode, the light passes through the polarizers 20 and 40 only once. Apparently, the conventional transflective LCD is not efficient in the reflective mode. The efficiency is important because the size, weight and battery life of portable instrumentation are heavily dependent upon the efficiency of the instrumentation""s display.
Accordingly, there exists a need in the art for a transflective liquid crystal display which overcomes, or at least reduces the above-mentioned problems of the prior art.
It is an object of the present invention to provide transflective LCDs which are more efficient and have low power consumption in the reflective mode.
To achieve the above listed and other objects, the present invention provides a liquid crystal display (LCD) characterized by having a dielectric multilayer formed inside an LCD cell so as to overcome, or at least reduce the above-mentioned problems of the prior art. The dielectric multilayer is composed of multiple layers of transparent dielectric materials with different refractive index. The LCD cell is sandwiched between two polarizers and comprises a first substrate and a second substrate being located facing each other with a liquid crystal layer therebetween. When the light traveling through the dielectric multilayer encounters material with different refractive index, a great part of the light is reflected while the other part is transmitted. The dielectric multilayer may be disposed between the first substrate and the liquid crystal layer or between the second substrate and the liquid crystal layer such that the light only passes through one of the polarizers in the reflective mode thereby increasing efficiency while meeting the need for low power consumption.
In a general aspect of the present invention, the first substrate is provided with a plurality of gate lines formed parallel to one another, a plurality of data lines formed parallel to one another vertically to the gate lines, a plurality of switching elements and pixel electrodes, and a passivation layer formed on the switching elements and pixel electrodes. The gate lines and the data lines are arranged to form a matrix of pixel regions with each of the pixel regions bounded by two adjacent gate lines and two adjacent data lines. The switching elements are formed at intersections of the gate lines and data lines, and the pixel electrodes are formed in the pixel regions. The passivation layer has a plurality of contact holes. The second substrate is provided with a light-shielding matrix, a plurality of color filters and a common electrode. A first polarizer, a retardation film, and a second polarizer on the retardation film are disposed outside of the LCD cell. A backlight is disposed behind the LCD cell.
According to a preferred embodiment of the present invention, the dielectric multilayer is directly formed on one surface of the first substrate, the gate lines are formed on the dielectric multilayer, and the light source is disposed behind the other surface of the first substrate. In this embodiment, the LCD cell may include an overcoat layer formed on the passivation layer. The overcoat layer has a plurality of contact holes to expose the contact holes of the passivation layer. The pixel electrodes are formed on the overcoat layer and electrically connected to the switching elements through the contact holes of the passivation layer and the contact holes of the overcoat layer.
According to another preferred embodiment of the present invention, the dielectric multilayer is formed on the gate lines as an insulating layer.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed between the switching elements and the pixel electrodes as a passivation layer. The dielectric multilayer has a plurality of contact holes, and the pixel electrodes are electrically connected to the switching elements through the contact holes of the dielectric multilayer.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed on an overcoat layer and the pixel electrodes. The overcoat layer is formed on the passivation layer and has a plurality of contact holes to expose the contact holes of the passivation layer. The pixel electrodes are formed between the overcoat layer and the dielectric multilayer, and the pixel electrodes are electrically connected to the switching elements through the contact holes of the passivation layer and the contact holes of the overcoat layer.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed on an overcoat layer and has a plurality of contact holes-formed corresponding to the contact holes of the passivation layer. The overcoat layer is formed between the passivation layer and the dielectric multilayer. The pixel electrodes are formed on the dielectric multilayer and electrically connected to the switching elements through the contact holes of the dielectric multilayer.
In the embodiments described above, the liquid crystal display may further includes a diffuser. The diffuser may be disposed inside or outside the LCD cell. Preferably, the diffuser is formed on the outer surface of the second substrate, and the retardation film is formed on the diffuser. Alternatively, the diffuser may be sandwiched between the common electrode and the color filters, and the retardation film is formed on the outer surface of the second substrate.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed on the inner surface of the second substrate, the first polarizer is formed on the outer surface of the second substrate, and the light source is behind the second substrate. In this embodiment, the liquid crystal display may further include a diffuser and a retardation film. The diffuser may be disposed inside or outside the LCD cell. Preferably, the diffuser is formed on the outer surface of the first substrate, and the retardation film is formed on the diffuser. Alternatively, the diffuser may be sandwiched between the common electrode and the color filters, and the retardation film is formed on the outer surface of the second substrate. Furthermore, the diffuser may be sandwiched between the pixel electrodes and the passivation layer, and the retardation film is directly formed on the outer surface of the first substrate.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed on an overcoat layer with an uneven surface (such as a corrugated surface) closest to the liquid crystal layer and the pixel electrodes. The overcoat layer is formed on the passivation layer and has a plurality of contact holes to expose the contact holes of the passivation layer. The pixel electrodes are sandwiched between the uneven surface of the overcoat layer and the dielectric multilayer, and are electrically connected to the switching elements through the contact holes of the passivation layer and the contact holes of the overcoat layer.
According to still another preferred embodiment of the present invention, the dielectric multilayer is formed on an overcoat layer with an uneven surface (such as a corrugated surface) closest to the liquid crystal layer. The overcoat layer is formed on the passivation layer and has a plurality of contact holes to expose the contact holes of the passivation layer. The dielectric multilayer has contact holes formed corresponding to the contact holes of the passivation layer. The pixel electrodes are formed on the dielectric multilayer and electrically connected to the switching elements through the contact holes of the dielectric multilayer.